// 双向节点类
class Dnode {
    // 构造器
    constructor (data = null, prev = null, next = null) {
        this.data = data
        this.prev = prev
        this.next = next
    }
    // 返回描述字符串
    toString () {
        return (this.data === null) ? 'head' : this.data
    }
}

module.exports = Dnode